Pabx transmission and impulsing circuit

ABSTRACT

A circuit arrangement is provided for two-way speech transmission and one-way impulse transmission in telecommunication exchanges. Transmission is across a transformer having first and second windings series connected to, but D.C. isolated from, first and second two-way speech paths respectively. Amplifier means is coupled through additional windings of the transformer to compensate for the attenuation of signals between the two speech paths due to natural transformer losses. A source of regulated D.C. power provides energy to the first two-way speech path and to an impulse repeater circuit coupled thereto. The repeater circuit provides substantially identical impulses to the second two-way speech path.

United States Patent 1191 Velik Feb. 26, 1974 PABX TRANSMISSION AND IMPULSING CIRCUIT lnventorz John Sidney Velik, Cremorne,

Australia Assignee: International Standard Electric Corporation, New York, NY.

[22] Filed: Jan. 20, 1972 Appl. No.: 219,241

[30] Foreign Application Priority Data Jan, 20, 1971 Australia PA 3759 US. Cl 179/170 R, 179/170 T 1m. 01. H04b 3/36 Field of Searchl79/17O A, 170 T, 170 D, 84 A;

[56] References Cited UNITED STATES PATENTS l/1965 Abbott 179/170 T 10/1969 Schlichte 330/26 OTHER PUBLICATIONS GE Transistor Manual; 1969; p. 228. 1

Basic Electronics; 1968; p. 68.

Primary ExaminerKathleen H. Claffy Assistant Examiner-Alan Faber Attorney, Agent, or FirmC. Cornell Remsen, Jr.; James B. Raden; Delbert P. Warner [5 7] ABSTRACT A circuit arrangement is provided for two-way speech transmission and one-way impulse transmission in telecommunication exchanges. Transmission is across a transformer having first and second windings series connected to, but D.C. isolated from, first and second two-way speech paths respectively. Amplifier means is coupled through additional windings of the trans former to compensate for the attenuation of signals between the two speech paths due to natural transformer losses. A source of regulated DC. power provides energy to the first two-way speech path and to an impulse repeater circuit coupled thereto. The repeater circuit provides substantially identical impulses to the second two-way speech path.

5 Claims, 1 Drawing Figure WN Mw E PATENTEDFB26 1974 Qgo BACKGROUND or THE INVENTION 1. Field of the Invention This invention relates to improvements in equipment of the P.A.B.X. type and in particular relates to speech transmission and impulsing repeater circuits for use in P.A.B.X. exchange lines or junctors.

2. Description of the Prior Art The specifications set by the Australian Post Office relating to transmission bridge and impulse repeater circuits for P.A.B.X. requirements have not been fully met by existing designs and indeed many of the performance parameters have fallen well short of the standards being sought. For example, previous circuits which use only capacitively-coupled speech paths obtained very poor longitudinal rejection figures due to poor longitudinal balance occasioned by differences in the impedances of the two sides of a circuit. With these prior circuits, if better figures were obtained, these were only achieved with the use of elaborate and costly transformers. The present invention, while also using a' transformer, permits the use of a simple and inexpensive type and achieves longitudinal balance not less than 53dB at 300 Hz and 70 dB at 3 kHz.

Again existing circuits have suffered from poor return loss characteristics, many failing badly at low frequencies and attempts to improve the performance figures have been at the expense of other characteristics of performance. The present invention has achieved a minimum return loss figure of dB, while insertion loss on a 600 ohm line has bettered the range 0.8 1.0 dB over 0.3 3.4 kHz.

Finally, the standards set for impulse repetition requires fixed limits on the width of both the make and break pulses which limits previous circuits have not been known to stringently meet. The present invention not only meets these limits, but also all the Australian Post Office specifications for transmission bridge and impulse repeater circuits.

SUMMARY OF THE INVENTION According to the invention a circuit arrangement for two-way speech transmission and one way impulse transmission in telecommunication exchanges is provided. The circuit employs a transformer having first and second windings series connected to, but DO isolated from, first and second two-way speech paths respectively. Amplifier means is provided in association with the transformer to substantially compensate the attenuation of signals between the two speech paths due to natural transformer losses. A source of regulated DC. power is used to supply the first two-way speech path. The DC. poweris also applied to impulse repeater circuit means which is connected with the first two way speech path and which upon the detection of impulses therefrom causes substantially identical impulses to be generated in the second two-way speech path.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the accompanying drawing which illustrates a preferred embodiment of the invention in the form of circuitry for extension-to-exchange dialling impulse and two-way speech transmission in P.A.B.X. applications.

DESCRIPTION OF THE PREFERRED EMBODIMENT A subscribers telephone subset is connected to the terminals A, B. Energy from a D..C. source is provided to the subset from the battery VI via a series standard regulator l (dotted enclosure) over terminal a of relay 2. The circuit of the series regulator is of a standard form and will be recognized by those skilled in the art and therefore requires no further description other than the fact that it comprises the familiar Darlington pair-zener diode arrangement. From terminal a of relay 2 current flows through the winding to terminal b, through diode D1 and the parallel combination of bias resistor R1 and the emitter-base circuit of transistor T1 to terminal d of inductor L1. Current thence flows from terminal 2 of inductor L1 to connection A, through the subset and back to connection B and through the secondary winding of L1, via terminals f and g, and finally to ground after passing through second windin g of relay 2 (terminals h and i). T1 is protected from large inverse voltages by the presence of diodes D1 and D2.

Dialling pulses from the extension are set up by opening and closing the current loop of the extension circuit and these pulses are detected by the series transistor T1 whose output is fed to the base of transistor T2 via resistor R2. T2 is chosen as a high gain transistor so that alternately it rapidly saturates and cuts-off in unison with the dialling pulse signal fed to its base. The emitter of T2 is held, say, at 4.5 volts by [both the zener diode Z1 and resistor R3 and, when the transistoris thrown into saturation, current flowsfrom the supply through the series-connectedwindings of the relay 3, resistor R4, collector-emitter path of T2 and to ground via Z1.

Resistor R5 is used to establish base bias on T2 as well as performing the load function for T1 while Z1 also serves to prevent conduction in T2 due to small leakage currents flowing across terminals A, B.

The dialling impulses driving T1 are substantially square-shaped this being ensured by the presence of capacitor C1 which cancels the effect of the back e.m.f. from L1 on the pulse detection integrity of Tl. Transformer 4 is D.C. isolated from the circuits described above by the series capacitor C2 which provides very low impedance to AC. signals for transmission along the speech path.

The dialling pulses are transmitted to the exchange from operation of relay 3 by its associated contact 5 via the closed contact 6 which is held closed by a relay (not shown) at the time the extension is brought into circuit. Contact 6 is opened in standard] P.A.B.X. function when the called extension answers thus removing the short across the speech circuit between C,D via transformer 4. Simultaneously, contact 5 is held closed completing the speech path to C. During the pulsing sequence spark quenching of contact 5 is achieved by the series combination of R6 and C3.

Operation of the circuit for speech transmission will now be examined.

In order to prevent speech loss in the extension, the battery supply is via an inductive element and hence the inclusion of L1. The audio signals from the extension are balanced and, as capacitor C2 has negligible impedance in the audio frequency spectrum, the full signal appears across the j-k winding of transformer 4, which being a 1:] transformer allows the signal to appear unattenuated across winding l-m. We are assuming at this stage that negligible transformer losses are involved. Transmission to the exchange is through the bridge network D3, D4, D5, D6 and closed contact to terminals C,D. Capacitor C4, like C2, offers near zero impedance at audio frequencies and serves to isolate the exchange direct current from transformer 4.

Return signals from the exchange follow the same path in the opposite direction. However, in order to provide a resistive component to the exchange on seizure, resistor R7 is located across the transformer side of the bridge network in series with contact 7 of relay 8 which in turn is in parallel with both R7 and contact 7. Upon detection of exchange current relay 8, which is very inductive and therefore presents negligible A.C. load to the line, will operate and remove R7 from the circuit by opening contact 7.

A purpose of the diode bridge D3-D6 is to aid in the prevention of large impulse voltages being generated from the inductive line and from relay 8 either during impulsing or when there is a test reversal on the exchange line. That is, the bridge ensures that D.C. through relay 8 is always in the same direction regardless of test reversals or an incorrectly poled exchange; the back e.m.f. would be considerably greater from relay 8 if the direction of DC. current were changed.

It should be noted that relay 2 is differentially wound as indicated in the drawing so that it will not operate unless the extension is unbalanced or an earth is applied to the terminals A,B. The provision of this relay is for certain P.A.B.X. functions, as will be recognized by telephone engineers, and does not, per se, form part of the inventive content.

Returning now to transformer 4 it will be realized that in practice it is not a lossless component and the assumption made earlier in this specification requires qualification.

In order to minimize transformer losses to negligible proportions, two extra windings are provided on 4. Signals detected across windings n-o are amplified by transistor T3 and presented to the collector load winding pq. The amplifier is designed to have low gain being sufficient to balance out the loss of the transformer. Biassing of T3 is achieved by resistors R8, R9, R10 with R10 unbypassed to provide substantial negative feed-back for the amplifier which is fed from battery V2 at a regulated supply of, say, 8 volts with the aid of the simple regulator network of resistor R11 and zener diode Z2. The remaining components in this transformer loss-reduction amplifier comprise zener diode Z3 and diode D6, which guard the transistor T3 against harmful voltages generated during impulsing or test reversal onthe exchange line, and capacitor C5 to prevent high frequency oscillation. It has been found that this type of amplifier arrangement coupled to transformer 4 closely approximates an ideal 1:1 lossless transformer suitable for P.A.B.X. application.

While the principles of the invention have been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

1. A circuit arrangement for two-way speech transmissions and one-way impulse transmission in telecommunication exchanges comprising a first two-way speech path, a second two-way speech path, a transformer having first and second windings series connected between said first and second two-way speech paths, means providing DC isolation between said transformer windings and said two-way speech paths, third and fourth windings coupled to said transformer, means coupling the third winding to amplifier means to provide a correction signal and means amplifying said correction signal to the fourth winding of the transformer to substantially compensate for the attenuation of signals between the two speech paths due to natural transformer losses, means coupling a source of regulated DC. power to the first two-way speech path, and an impulse repeater circuit connected to said first twoway path and to said source of DC power to respond to impulses in said first two-way path to cause changes including closure of relay contacts in the second two-way speech path causing substantially identical impulses to appear in the second two-way speech path.

2. A circuit arrangement as claimed in claim 1, in which the means coupling a source of regulated DC power to the first two-way speech path includes relay windings coupled in series between the DC power source and the first two-way speech path.

3. A circuit arrangement as claimed in claim 1, in which the impulse repeater circuit includes a first transistor coupled to respond to DC dial pulses and supply operate signals, a relay, a second transistor coupled to said first transistor and to said relay to respond to said operate signals to provide control signals to said relay and cause operation of contacts of said relay in said second two-way speech path to thereby provide pulses to said second two-way speech path.

4. A circuit arrangement as claimed in claim 1, in which the source of regulated DC power includes a battery coupled through a Darlington regulator circuit and through elements of a dial pulse detector circuit to said first two-way speech path.

5. A circuit arrangement as claimed in claim 1, in which said means providing DC isolation includes first and second capacitors, means coupling said first capacitor in series between the first winding of said transformer and the first two-way speech path, and means coupling said second capacitor in series between the second winding of said transformer and the second 

1. A circuit arrangement for two-way speech transmissions and one-way impulse transmission in telecommunication exchanges comprising a first two-way speech path, a second two-way speech path, a transformer having first and second windings series connected between said first and second two-way speech paths, means providing DC isolation between said transformer windings and said two-way speech paths, third and fourth windings coupled to said transformer, means coupling the third winding to amplifier Means to provide a correction signal and means amplifying said correction signal to the fourth winding of the transformer to substantially compensate for the attenuation of signals between the two speech paths due to natural transformer losses, means coupling a source of regulated D.C. power to the first two-way speech path, and an impulse repeater circuit connected to said first two-way path and to said source of DC power to respond to impulses in said first two-way path to cause changes including closure of relay contacts in the second two-way speech path causing substantially identical impulses to appear in the second two-way speech path.
 2. A circuit arrangement as claimed in claim 1, in which the means coupling a source of regulated DC power to the first two-way speech path includes relay windings coupled in series between the DC power source and the first two-way speech path.
 3. A circuit arrangement as claimed in claim 1, in which the impulse repeater circuit includes a first transistor coupled to respond to DC dial pulses and supply operate signals, a relay, a second transistor coupled to said first transistor and to said relay to respond to said operate signals to provide control signals to said relay and cause operation of contacts of said relay in said second two-way speech path to thereby provide pulses to said second two-way speech path.
 4. A circuit arrangement as claimed in claim 1, in which the source of regulated DC power includes a battery coupled through a Darlington regulator circuit and through elements of a dial pulse detector circuit to said first two-way speech path.
 5. A circuit arrangement as claimed in claim 1, in which said means providing DC isolation includes first and second capacitors, means coupling said first capacitor in series between the first winding of said transformer and the first two-way speech path, and means coupling said second capacitor in series between the second winding of said transformer and the second two-way speech path. 